The present invention generally relates to transformers such as a transformer for heavy current, a current transformer (CT), a potential transformer (PT) and a transformer for weak current and more particularly, to a transformer including a winding wound annularly a predetermined number of times and a pair of wound cores each obtained by winding an electromagnetic magnetic plate around the winding a predetermined number of times, and a coil bobbin for use in the transformer.
For example, Japanese Patent Laid-Open Publication No. 5-226168 (1993) filed by the assignee assigned by the present inventors discloses this kind of the transformer as shown in FIGS. 33 and 34. In this known transformer, an outer periphery 2a of a winding 2 obtained by annularly winding a conductor a predetermined number of times is coated by an insulating member (not shown) such as an insulating tape and an insulating sheet and each of a pair of wound cores 3 is obtained by winding an electromagnetic steel plate around the winding 2. In FIGS. 33 and 34, reference numeral 4 denotes a spacer and reference numeral 5 denotes a magnetic shunt core. In order to produce the winding 2, a conductor is wound around a split winding form (not shown) made of Bakelite or the like and then, the winding is coated by the insulating member by removing the winding form.
Meanwhile, the wound core 3 is formed by steps shown in FIGS. 35A to 35D. Initially, after a steel plate coil 7 obtained by winding a long belt-like electromagnetic steel plate 8 so as to have inside diameter coincident with outside diameter of the winding 2 has been annealed, the electromagnetic steel plate 8 disposed at an outermost portion of the steel plate coil 7 is passed between a pair of core winding portions 2b as shown in FIG. 35A. Then, as shown in FIG. 35B, a rear end portion 8a of the electromagnetic steel plate 8 is temporarily attached to an outer periphery of the steel plate coil 7 such that a large ring 9 having diameter larger than outside diameter of the steel plate coil 7 is formed by the electromagnetic steel plate 8. Furthermore, the steel plate coil 7 is rotated by driving rollers 11 and 12 such that the electromagnetic steel plate 8 is fed to the large ring 9 as shown by the arrow A. When rotation of the steel plate coil 7 is continued, whole of the electromagnetic steel plate 8 constituting the steel plate coil 7 is fed to the large ring 9 as shown in FIG. 35C. Since the electromagnetic steel plate 8 has elasticity, a force B for reducing diameter of the large ring 9 is applied to the large ring 9. When not only the roller 11 is drawn from the large ring 9 but the temporary attachment referred to above is cancelled, diameter of the large ring 9 is reduced as shown in FIG. 35D so as to tighten one of the core winding portions 2b such that the wound core 3 is formed.
However, the above mentioned known transformer has the following drawbacks. Namely, for winding a conductor 10 to the winding 2 in the transformer including the wound cores 3, if an alignment winding method of FIG. 36 in which the neighboring conductors 10 in each layer are held in close contact with each other and the conductors 10 in the neighboring layers deviate laterally from each other through a radius r of the conductor 10 such that gaps among the conductors 10 are minimized is employed and the outer periphery 2a of the winding 2 is brought into close contact with the wound cores 3, heat produced in the conductors 10 by electric current flowing therethrough, i.e., resistance loss is efficiently dissipated through the wound cores 3. Therefore, in order to reduce rise of temperature of the winding 2, it is preferable that the winding 2 is formed by the alignment winding method and the outer periphery 2a of the winding 2 is brought into close contact with the wound cores 3.
However, in case the winding 2 is produced by using the split winding form as described above, the winding 2 is readily deformed once the winding form has been removed. As a result, it is difficult to maintain the conductors 10 in a state of FIG. 36 in which the conductors 10 have been closely wound by the alignment winding method. Furthermore, since the winding 2 is readily deformed as described above, it is difficult to maintain a state in which cross-sectional shape of the winding 2 coincides with inside diameter of the wound cores 3 and thus, it is impossible to hold the outer periphery 2a of the winding 2 and inside diameter of the wound cores 3 in close contact with each other. Accordingly, in the known transformer referred to above, rise of temperature of the winding 2 caused by heat produced in the winding 2 cannot be prevented effectively and it is difficult to make the transformer compact.
Meanwhile, if insulating property of the winding 2 and the wound cores 3 deteriorates, performance of the transformer drops. In the construction in which the winding 2 is coated by the insulating member as described above, the insulating member may be damaged through contact between the outer periphery 2a of the winding 2 and the wound cores 3 in the step of FIG. 35C for feeding the electromagnetic steel plate 8 to the large ring 9 and through contact between an end of the electromagnetic steel plate 8 disposed at an innermost portion of the large ring 9 and the outer periphery 2a of the winding 2, thereby resulting in deterioration of insulating property of the winding 2 and the wound cores 3.
Furthermore, since it is difficult to make cross-sectional shape of the winding 2 coincident with inside diameter of the wound cores 3 as described above, such a case may happen in which inside diameter of the steel plate coil 7 is different from that of the wound cores 3 wound around the core winding portions 2b. In this case, residual strain is produced in the electromagnetic steel plate 7 constituting the wound core 3, thus resulting in deterioration of magnetic characteristics of the electromagnetic steel plate 8.
On the other hand, Japanese Utility Model Laid-Open Publication No. 54-177512 (1979) and Japanese Patent Laid-Open Publication No. 2-165610 (1990) disclose coil bobbins around which a conductor is wound and on which wound cores are formed. As shown in FIGS. 37 and 38, the former prior art document discloses a coil bobbin 19 constituted by outer and inner frames 17 and 18 formed, on the outer periphery, with grooves 17a and 18a for forming windings 16A and 16B by winding the conductor 10. Meanwhile, as shown in FIGS. 39 and 40, a transformer disclosed in the latter prior art document includes a first bobbin 23 constituted by primary and secondary frames 21 and 22 and a second bobbin 24 surrounding the first bobbin 23. The conductor 10 is wound a predetermined number of times around grooves 21a and 22a formed on the primary and secondary frames 21 and 22, respectively so as to form windings 25A and 25B. Meanwhile, a pair of wound cores 26 are provided on an outer periphery of the second bobbin 24.
In the above two coil bobbins, if outside diameter of the coil bobbin is made coincident with inside diameter of the wound cores when the winding is formed on the coil bobbin, the winding and the wound cores can be brought into close contact with each other. Meanwhile, if the coil bobbin is used, the wound core and the electromagnetic steel plate are held out of contact with each other when the wound core is wound around the winding, so that damage to the winding can be prevented. However, even if the coil bobbin is used, the following problem arise. Initially, in the known coil bobbins, since cross-sectional shape of the grooves 17a, 18a, 21a and 22a is semicircular, it is difficult to closely wind the conductor 10 by the alignment winding method. Hence, the windings 16A, 16B, 25A and 25B are set to a so-called disorderly winding state in which a number of gaps are formed among the conductors 10. Therefore, heat produced in the conductors 10 cannot be dissipated efficiently and thus, it is impossible to effectively reduce rise of temperature of the winding. Especially, in the outer frame 17 of FIG. 37 and the primary and secondary frames 21 and 22 of FIG. 40, since width W of their opening is smaller than width of the grooves 17a, 21a and 22a, it is extremely difficult to wind the conductor 10 by the alignment winding method.
Meanwhile, in the above known coil bobbins, since cross-sectional shape of a whole outer periphery of the coil bobbin is circular at its portion for winding the wound core therearound, friction between the coil bobbin and the electromagnetic steel plate 8 constituting the wound core is large when the wound core is formed. Thus, unless diameter of the large ring 9 shown in FIGS. 35B and 35C is formed large, it is difficult to wind the electromagnetic steel plate 8 around the coil bobbin smoothly. However, if diameter of the large ring 9 is increased, dimensional difference between the steel plate coil 7 and the large ring 9 increases, so that a portion of the electromagnetic steel plate 8, which is deformed beyond its elastic limit, is made larger and thus, magnetic characteristics of the wound core deteriorate.